Why is my fire simulation so blocky in Cycles rendered view?
I've animated flames in blender a few times in the past, and one thing I learned is that if you want the flames to look realistic.  You usually have to set the resolution divisions REALLY high.  Around 600 or even 700 division should make it look really nice. Or maybe 400-500 if you're not going for super realistic.  Yes, it will drastically increase your rendering time, but that's the price you have to pay sometimes.